1. INTRODUCTION

This paper reviews internal processes of secular evolution in disk galaxies.
We concentrate on one important consequence: the buildup of dense
central components that look like classical, i.e., merger-built bulges
but that were made slowly by disks out of disk material. These are
called pseudobulges. Our discussion updates a review by
Kormendy (1993).

The relative importance of the different physical processes of galaxy
evolution (Figure 1) changes as the Universe
expands. Rapid processes that happen in discrete events are giving way
to slow, ongoing processes.

Figure 1. Morphological box
(Zwicky 1957)
of processes of galactic evolution updated from
Kormendy (1982a).
Processes are divided vertically into fast (top) and slow
(bottom). Fast evolution happens on a free-fall ("dynamical")
timescale, tdyn ~
(G)-1/2, where
is the
density of the object produced and G
is the gravitational constant. Slow means many galaxy rotation periods.
Processes are divided horizontally into ones that happen purely
internally in one galaxy (left) and ones that are driven by
environmental effects such as galaxy interactions (right).
The processes at center are aspects of all types of galaxy evolution.
This paper is about the internal and slow processes at lower left.

At early times, galactic evolution was dominated by a combination of
dissipative collapse
(Eggen, Lynden-Bell
& Sandage 1962;
Sandage 1990)
and mergers
(Toomre 1977a)
of galaxies that virialized out of the density fluctuations of cold dark
matter. These are the top processes shown in
Figure 1. The evolution timescale was short,
tdyn ~ (1 /
G)1/2, where
is the mean
density and G
is the gravitational constant. The processes were violent. Many
present-day galaxies owe their properties to this violence. Because
mergers scramble disks and induce dissipation and starbursts, they are
thought to make classical bulges and elliptical galaxies. We do not
review classical bulges other than to contrast them to pseudobulges.
Most work on galaxy evolution in the past 25 years has concentrated on
hierarchical clustering and mergers. As the Universe expands, and as
galaxy clusters virialize and acquire large internal velocities,
mergers get less common
(Toomre 1977a,
Conselice et
al. 2003).

In the distant future, internal secular processes will become dominant.
These are defined to be slow processes, i.e., ones that have timescales
much longer than tdyn. To be interesting, they must
operate over long times. Some secular processes, such as disk heating
via stellar enounters with molecular clouds, are well known
(Spitzer &
Schwarzschild 1951,
1953).
But star-star relaxation is too slow to be important almost everywhere
in almost every galaxy. Therefore, relevant secular processes generally
involve the interactions of individual stars or gas clouds with
collective phenomena such as bars, oval distortions, spiral structure,
and triaxial dark matter halos. Also important are the interactions of
these collective phenomena with each other. Given that hierarchical
clustering continues today, has there been time for slow processes to
be important? For secular evolution to have a significant effect, a
galaxy must be free of major mergers for a long time, because merger
violence erases the signature of secular processes. Hierarchical
clustering results in so many mergers that one might guess that secular
processes are relatively unimportant. A clue that this is frequently
not the case is provided by galaxies with superthin - and fragile - disks
but apparently no bulges (e.g.,
Matthews, Gallagher
& van Driel 1999b;
Freeman 2000;
van der Kruit et
al. 2001).
They show that many galaxies have suffered no major merger violence
since the onset of star formation in the disk
(Tóth &
Ostriker 1992).
Therefore there has been time for secular evolution to be important in
at least some galaxies. Given that mergers make bulges and ellipticals,
these tend to be late-type galaxies. Actually, because the latest-type
galaxies are (pseudo)bulgeless, secular evolution is likely to be most
important in intermediate-late-type galaxies, i.e., Sbcs. But even some
S0 and Sa galaxies contain pseudobulges. Secular processes have
received less attention than galaxy mergers.

Theory
and observations point to a variety of processes that redistribute
energy in disks. In Section 2, we review
evidence that bars and ovals
rearrange disk gas into outer rings, inner rings, and central mass
concentrations. The resulting star formation produces a central stellar
subsystem that has the high density and steep density gradient of a
bulge but that was not formed by galaxy mergers.

Secular
evolution is not confined to barred and oval galaxies. Bars can
self-destruct by building up the central mass concentration, so secular
evolution may have happened even if no bar is seen today. Global spiral
structure also makes galaxies evolve, albeit more slowly than do bars.

These processes are manifestations of very general dynamical principles.
Disks spread in radius - the inner parts shrink and the outer parts
expand - because this lowers the total energy for fixed total angular
momentum
(Lynden-Bell &
Kalnajs 1972,
Tremaine 1989).
Two-dimensional spreading by angular momentum transport is as
fundamental for rotation-dominated disks as is three-dimensional
spreading by energy transport in the core collapse of ellipsoidal
systems dominated by random motions. The reason is the same, too.
Self-gravitating systems have negative specific heats, so increasing
the central density by flinging away the periphery lowers the total
energy
(Lynden-Bell &
Wood 1968,
Binney & Tremaine
1987).
What makes evolution important in some systems and not in others? The
determining factor is whether any evolution process is fast enough.
Core collapse requires short relaxation times. Galaxy disks have long
relaxation times, so their evolution is interesting only if they have
an alternative to relaxation. Nonaxisymmetries provide the engine for
rapid evolution.

We argue here that pseudobulges are one result
of this evolution. Of key importance is the observation that they
retain some memory of their disky origin. We review this subject in
detail (Section 4), because it is central
to any conclusion that evolution has happened and because it is the only
way that we can recognize pseudobulges.

Next we review observations of gas
content and star-formation rates. From these, we estimate pseudobulge
growth times. These prove to be in good agreement with stellar
population ages. So the picture of pseudobulge growth from rearranged
disk gas is internally consistent.

Our purpose is to connect up
a large number of apparently disparate results into a well-developed
and (as we hope to show) a well-supported paradigm. Still, many
questions remain unanswered. In particular, we need a better
understanding of the relative importance of mergers and secular
evolution as a function of galaxy type and luminosity. We hope that
this review will provide a concrete context that will allow efficient
progress in this subject.

What do we mean by a bulge? The answer shows why, for some galaxies, we
use the term pseudobulge.

Renzini (1999)
clearly states the canonical interpretation of Hubble-Sandage-de
Vaucouleurs classifications: "It appears legitimate to look at bulges
as ellipticals that happen to have a prominent disk around them [and]
ellipticals as bulges that for some reason have missed the opportunity
to acquire or maintain a prominent disk." We adopt this point of view.
However, as observations improve, we discover more and more features
that make it difficult to interpret every example of what we used to
call a bulge as an elliptical living in the middle of a disk. This
leads authors to agonize: "Are bulges of early-type and late-type
spirals different? Are their formation scenarios different? Can we talk
about bulges in the same way for different types of galaxies?"
(Fathi & Peletier
2003).

We conclude that early- and late-type galaxies generally do make their
dense central components in different ways. This is not recognized in
classical morphology, because it defines classification
bins - deliberately and with good reason - without physical interpretation.
Sandage & Bedke
(1994)
describe how, in the early stages of investigating a subject, a
classifier should look for "natural groups"
(Morgan 1951)
of objects with similar features. Sandage emphasizes that it is
important not to be led astray by preconceptions: "The extreme
empiricist claims that no whiff of theory may be allowed into the
initial
classification procedures, decisions, and actions." Nevertheless, some
choice of which features to consider as important and which to view as
secondary must be made. After all, the goal is to understand the
physics, and the exercise is useful only if classification bins at
least partly isolate unique physics or order galaxies by physically
relevant parameters. The Hubble-Sandage-de Vaucouleurs classification
scheme has done these things remarkably well.

However, it is
reasonable to expect that improved understanding of galaxies will show
that the classification missed some of the physics. Also, some features
of galaxies could not be observed well enough in the photographic era
to be included. These include high-surface-brightness disky
substructures in galaxy centers. Consistent with physical morphology as
discussed in
Kormendy (1982a),
we wish to distinguish components in galaxies that have different origins.

At
the level of detail that we nowadays try to understand, the time has
passed when we can make effective progress by defining morphological
bins with no guidance from a theory. Disks, bulges, and bars were
different enough that we could do this. Afterward, robust conclusions
could be reached, e.g., about the relative timescales of collapse and
star formation
(Eggen, Lynden-Bell
& Sandage 1962).
But even inner rings and spiral arms - which are not subtle - do not scream
the appropriate message, which is that spiral arms are details that
would disappear quickly and without a trace if the driving mechanism
switched off, whereas we will see that rings are a permanent
rearranging of disk material. Inner rings are, in this sense, more
fundamental than spiral arms. Years ago, people commonly reacted badly
to a classification as complicated as (R)SB(r)b
(de Vaucouleurs et
al. 1991).
The reason, we believe, was that the phenomenology alone did not sell
itself. People did not see why this level of detail was important. Now,
we will show that every letter in the above classification has a
clear-cut meaning in terms of formation physics. This is the goal of
physical morphology.

We adopt the view that bulges are ellipticals living in the middle of
disks. Ellipticals form via mergers
(Toomre 1977a,
Schweizer 1990).
Therefore, we do not use the term bulge for every central component
that is in excess of the inward extrapolation of an exponential fitted
to the disk brightness profile. If the evidence suggests that such a
component formed by secular processes, we call it a pseudobulge. In
practice, we cannot be certain about formation mechanisms. Therefore,
if the component in question is very E-like, we call it a bulge, and if
it is disk-like, we call it a pseudobulge. Intermediate cases are
discussed in Sections 4,
7, and 9.1.

Finally, we
comment on one of the biggest problems in this subject. It is
exceedingly easy to get lost in the details. Many authors interpret
observations or simulations in much more detail than we do here. For
example, it is common for observers to distinguish nuclei, nuclear
bars, nuclear disks, nuclear spiral structure, exponential bulges, boxy
bulges, and star-formation rings. We discuss all these features,
because they are central to the developing picture of what secular
evolution can accomplish. But we consider them all to be features of
pseudobulges, because the evidence is that they are all built by
secular evolution out of disk material. In the same way, global spiral
structure, flocculent spiral structure, and the absence of spiral
structure in S0 galaxies are all features of disks.